West Nile Virus (WNV) Information for Healthcare Professionals 

Healthcare Professionals: Epidemiology | Transmission | Clinical Description/Case Definition | Reporting | Resources

West Nile virus (WNV) was first isolated and identified in 1937 in a febrile person in the West Nile district of Uganda. Prior to 1999, the virus was found only in the Eastern Hemisphere, with wide distribution in Africa, Asia, the Middle East, and Europe. There were infrequent reports of human outbreaks, mainly associated with mild febrile illnesses, in Israel and Africa. These were mostly in groups of soldiers, children, and healthy adults. One notable outbreak in Israeli nursing homes in 1957 was associated with severe neurologic disease and death.

Arkansas surveillance has documented persons with illness caused by WNV each year since 2002: Since the emergence of WNV in 2002, Arkansas Department of Health has tracked reported cases of WNV, including cases of West Nile Fever and cases of Neuroinvasive disease (including Meningitis, Encephalitis, and Meningoencephalitis). Click here for detailed summary of West Nile Virus activity in Arkansas

Peak incidence of human disease in Arkansas occurs in August.

Predicting the temporal characteristics of future WNV transmission seasons based on limited reports available to date is not possible. Despite this limitation, enhanced passive surveillance for human cases should be encouraged beginning in early spring and continuing through the fall until mosquito activity ceases because of cold weather.


  • West Nile virus (WNV) is a single-stranded RNA virus of the family Flaviviridae, genus Flavivirus.
  • WNV is a member of the Japanese encephalitis virus antigenic complex, which includes several medically important viruses associated with human encephalitis: Japanese encephalitis, St. Louis encephalitis, Murray Valley encephalitis, and Kunjin, an Australian subtype of WNV. The close antigenic relationship of the flaviviruses, particularly those belonging to the Japanese encephalitis complex, accounts for the serologic cross-reactions observed in the diagnostic laboratory.
  • For unknown reasons, deaths among birds from WNV infection have occurred only in the United States, Israel, Canada, and Mexico. 
  • Since 1999, very few genetic changes have occurred in the WNV strains circulating in the United States.

Primary Mode of Transmission

  • WNV is maintained in nature in a transmission cycle that involves primarily birds and mosquitoes.
  • The main route of human infection is through the bite of an infected mosquito.
  • Mosquitoes become infected when they feed on infected birds, which may circulate the virus in their blood for a few days. Infectious mosquitoes carry virus particles in their salivary glands and infect susceptible bird species during blood-meal feeding. Bird reservoirs will sustain an infectious viremia for 1 to 4 days after exposure after which the hosts that survive develop life-long immunity.
  • It is unknown exactly how the virus survives dry seasons or winters. One proven over-wintering mechanism is in infected adult female Culex mosquitoes, which hibernate during the winter.
  • People, horses, and most other mammals are not known to commonly develop infectious-level viremias and thus are probably "dead-end" or incidental hosts.
  • Persons should avoid bare-handed contact with dead animals and use gloves or double plastic bags when collecting bird carcasses for disposal.
  • There is no documented evidence of animal-to-person transmission of WNV apart from mosquitoes.

Alternative Modes of Transmission

It is important to note that these other methods of transmission represent a very small proportion of cases.

  • Transplanted organs
  • Blood transfusions
  • Breastmilk
  • Transplacental (mother-to-child)
  • Occupational exposure

Clinical Description and Case Definition

Arkansas Department of Health’s case definition for Arboviral neuroinvasive and non-neuroinvasive diseases agrees with the current CDC case definition. Click here to view the CDC WNV Information and Guidance for Clinicians.

Reporting Information

The Arkansas Department of Health partners with healthcare providers in the medical community to perform surveillance of WNV. The Arkansas Department of Health (ADH) is an active participant in the National Electronic Disease Surveillance System (NEDSS). Developed by the Centers for Disease Control and Prevention (CDC), NEDSS is a system to improve the public health monitoring of diseases.

TBD is a disease of public health significance and is to be reported to the Arkansas Department of Health within 24 hours of diagnosis. Reports should include:

  1. The reporter’s name, location and phone number;
  2. The name and onset date of the disease;
  3. The patient’s name, address, phone number, age, sex and race;
  4. The attending physician's name, location and phone number;
  5. Any pertinent clinical, laboratory, and treatment information.

Report by Fax to 501-661-2428; 24 hr. answering machine 800-482-8888; in person to 501-661-2893.

Instructions for Reporting Communicable Diseases
Communicable Disease Reporting Form

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